The macula is the area of the retina which in humans is responsible for central vision. It is centered around the area where the visual axis meets the retina and extends radially outward a distance about 2.75 mm or one-tenth of one inch. The macula is divided into an inner and outer macula. The outer macula is an annular ring surrounding the inner macula. Although small, the macula has the highest degree of visual acuity. Diseases that disrupt the functioning of the macula substantially affect the vision of a patient with one of those diseases. Once such disease is age related macular degeneration (AMD). It has been found that this occurs in approximately 20 percent of the population above the age of 65 and it is a leading cause of visual impairment in industrialized countries. A portion of the macula has a layer of pigmented epithelial known as the retinal pigment epithelium (RPE). It is believed that macular degeneration is caused by disruption, deterioration, or death of the retinal pigment epithelium cells. AMD is usually diagnosed by collection of clinical findings. These include drusen, retinal pigmented epithelial disturbance, which includes pigment clumping, RPE detachment, geographic atrophy, sub-retinal neovascularization, or disciform scar. The presence of drusen alone do not seem to be directly associated with vision loss and, indeed, up to 85 percent of people are found to have drusen upon ocular examination.
Although the exact cause of macular degeneration is unknown, it is believed the onset and progression of macular degeneration is accelerated by toxicity from free radicals and oxidizers. It is believed that oxidants including singlet oxygen and other free radicals can occur as result of phototoxicity. That is to say, the presence of light can react with the presence of oxygen to produce singlet oxygen, a free radical, leading to potential damage from chemical reactions between the physiological structures that constitute the macula and these oxidizing agents including singlet oxygen.
It is known that naturally occurring substances that humans intake with a normal diet can serve as antioxidants and free radical scavengers, possibly reducing or eliminating tissue damage from oxidation or the presence of free radicals in those tissues. More specifically, it has been reported that high-dose supplementation with Vitamin C and E, beta-carotene, and zinc had a clinically significant affect on the occurrence and progression of macular degeneration and vision loss (see Age-Related Eye Disease Study Research Group: “A Randomized Placebo-Controlled Clinical Trial of High-Dose Supplementation of Vitamin C and E, Beta-carotene and Zinc for Age Related Macular Degeneration and Vision Loss: AREDS Regular Report No. 8”, Archives of Ophthalmology, Volume 119, No. 10 Oct. 2001, pages 1417-1436). In the AREDS study, a nutritional substance was evaluated. This substance contained 500 mgs of Vitamin C, 400 IUs of Vitamin E, 15 mgs of beta-carotene, 80 mgs of Zinc, and 2 mgs of copper. Vitamins C, E, and beta carotene are known antioxidants. Vitamin C and beta-carotene are water soluble. On the other hand, Vitamin E is a fat soluble vitamin. For water soluble vitamins, the excess is excreted from the body, but fat soluble vitamins are stored and can accumulate in the liver, fat, and muscle tissues. There have been some negative reactions associated with mega doses of Vitamin E, which includes headache, fatigue, diplopia, and diarrhea. Animal studies have shown that high doses of Vitamin E may interfere with absorption of other fat soluble vitamins. Consequently, there is a concern that large doses of Vitamin E consumed over weeks or months may result in deficiencies of Vitamins D, A, and K.
Vitamin A has been shown to slow the progression of retinitis pigmentosa (RP). However, Vitamin E has shown to be potentially harmful. Vitamin E may interfere with the absorption of Vitamin A, hence, can reduce the effectiveness of Vitamin A intake in slowing the progression of RP.
Vitamin C is a well-known water soluble antioxidant. Humans must consume sources of Vitamin C to meet the Vitamin C requirements for normal physiological functioning. The recommended daily allowance (RDA) for Vitamin C in the form of ascorbic acid is 60 mgs. However, large daily doses of Vitamin C have been taken over many years with no undesirable effects. Intakes of 1000 mgs or more have been consumed daily without adverse affects.
Beta-carotene is a proform of Vitamin A. Beta-carotene is a water soluble orange pigment found in many vegetables. In the body beta-carotene is converted to Vitamin A with efficiency approximately 50 percent. The RDA of Vitamin A is 5000 IUs. It is understood that beta-carotene is a highly effective antioxidant. There have been adverse effects associated with high doses of beta carotene for people with myocardial infarctions. There is also a reported increase of lung cancer among smokers who receive upwards of 20 mgs a day of beta-carotene. Beta-carotene is a carotenoid. Another naturally occurring carotenoid is lutein. Lutein is the primary carotenoid present in the macula. It is believed that lutein may act as a filter to protect the macula from potentially damaging forms of light. There is evidence to suggest that people who eat more lutein containing foods appear to be at a lower risk of macular degeneration than the population at large. One study has found that adults with the highest dietary intake of lutein had a 57 percent decreased risk of macular degeneration compared with people with the lowest intake. For this reason, lutein may be used in a dietary supplement to supplement or substitute for beta-carotene. Another naturally occurring carotenoid is zeaxanthin. Like lutein and beta-carotene, zeaxanthin is an antioxidant found in the retina of healthy eyes. It is believed that the presence of these carotenoids in the retina of healthy eyes tends to protect against damage from phytotoxicity, free radicals, and other oxidizing agents. Consequently, it has been found that dietary supplements containing an appropriate combination of the carotenoids, beta-carotene, lutein, and/or zeaxanthin may be necessary for healthy eyes.
In addition to the carotenoids, a variety of other compounds have been found to have antioxidant qualities. For example, alpha-lipoic acid may enhance the potency of other antioxidants in the body. Phenolic compounds can also serve as useful antioxidants. Some of these phenolic compounds are found naturally in grape seeds. The anthocyanosides are also useful antioxidants that are found naturally in bilberry fruit.
Zinc is one of the essential minerals that forms a part of many enzymes which are physiologically important, including those that are involved in digestion, metabolism, reproduction, and wound healing. High doses of Zinc have been associated with anemia. It is believed that the anemia associated with high Zinc intake is attributable to Copper deficiency. Consequently, it is usually thought desirable for a nutritional supplement that includes high doses of Zinc that there also be included some intake of Copper. Copper, like Zinc, is important in various bodily enzymes. Therefore, adding Copper to a nutritional supplement to accompany Zinc will reduce the risk of Zinc intake leading to a Copper deficiency. Even though it is widely understand that nutritional supplements containing antioxidants in various forms and mineral supplements may be useful in promoting retinal health and preventing age-related retinal diseases, work still remains to be done to determine exactly what balance of nutritional supplements in what proportions and what ingredients will provide the best protection.